There are various types of coordinate-capturing system using a tablet and an input pen, including a pressure-sensitive type, an electrostatic-coupling type, an electromagnetic-coupling type, an ultrasonic in-medium propagation (surface acoustic wave) type, and an ultrasonic aerial-propagation type.
In each of the above conventional types except the ultrasonic aerial-propagation type, a sensor is built into a medium in the shape of a plate. Accordingly, they become too cumbersome to carry since their dimensions are determined by their input surfaces.
On the other hand, the ultrasonic aerial-propagation type has not the constraint of the input surface, and any surface can be used as the input surface. Japanese Unexamined Utility Model Publication No. 64-7340 discloses determining a position of a sounding object by receiving a sound wave emitted from the object by at least three microphones disposed on the same plane, determining at least two hyperbolic curves on each of which two associated microphones lie, on the basis of difference in timing of reception of the sound wave between the two associated microphones, and determining a point of intersection of at least two determined hyperbolic curves (refereed to as “3-ultrasonic-wave type”). U.S. Pat. No. 4,814,552 discloses a different method for determining coordinates of a position of a pen. In this method, an ultrasonic signal for position sampling and an electromagnetic signal for timing detection of emission of the ultrasonic signals are emitted from the pen, and are received by two ultrasonic receivers and one electromagnetic signal receiving element respectively. Then, time delays between reception of the electromagnetic signal and receptions of the ultrasonic sampling signal by the two ultrasonic receivers are measured to calculate distances between the pen and the ultrasonic receivers taking account the propagation speed of the ultrasonic signal, thereby determining the position of the input pen by trigonometry (referred to as “2-ultrasonic-wave+electromagnetic-wave type”).